A need exists for toners which melt at lower temperatures than a number of toners now commercially used with certain copying and printing machines. Temperatures of approximately 160.degree.-200.degree. C. are often selected to fix toner to a support medium such as a sheet of paper or transparency to create a developed image. Such high temperatures may reduce or minimize the life of certain fuser rolls such as those made of silicone rubbers or fluoroelastomers (e.g., Viton.RTM.), may limit fixing speeds, may necessitate larger amounts of power to be consumed during operation of a copier or printer such as a xerographic copier which employs a method of fixing such as, for example, hot roll fixing.
Toner utilized in development in the electrographic process is generally prepared by mixing and dispersing a colorant and a charge enhancing additive into a thermoplastic binder resin, followed by micropulverization. As the thermoplastic binder resin, several polymers are known including polystyrenes, styrene-acrylic resins, styrene-methacrylic resins, polyesters, epoxy resins, acrylics, urethanes and copolymers thereof. As the colorant, carbon black is utilized often, and as the charge enhancing additive, alkyl pyridinium halides, distearyl dimethyl ammonium methyl sulphate, and the like are known.
Toner can be fixed to a support medium such as a sheet of paper or transparency by different fixing methods. A fixing system which is very advantageous in heat transfer efficiency and is especially suited for high speed electrophotographic processes is hot roll fixing. In this method, the support medium carrying a toner image is transported between a heated fuser roll and a pressure roll, with the image face contacting the fuser roll. Upon contact with the heated fuser roll, the toner melts and adheres to the support medium forming a fixed image.
Fixing performance of the toner can be characterized as a function of temperature. The lowest temperature at which the toner adheres to the support medium is called Cold Offset Temperature (COT), and the maximum temperature at which the toner does not adhere to the fuser roll is called the Hot Offset Temperature (HOT). When the fuser temperature exceeds HOT, some of the molten toner adheres to the fuser roll during fixing and is transferred to subsequent substrates containing developed images, resulting for example in blurred images. This undesirable phenomenon is called offsetting. Between the COT and HOT of the toner is the Minimum Fix Temperature (MFT) which is the minimum temperature at which acceptable adhesion of the toner to the support medium occurs, that is, as determined by for example a creasing test. The difference between MFT and HOT is called the Fusing Latitude.
The hot roll fixing system described above and a number of toners presently used therein exhibit several problems. First, the binder resins in the toners can require a relatively high temperature in order to be affixed to the support medium. This may result in high power consumption, low fixing speeds, and reduced life of the fuser roll and fuser roll bearings. Second, offsetting can be a problem. Third, toners containing vinyl type binder resins such as styrene-acrylic resins may have an additional problem which is known as vinyl offset. Vinyl offset occurs when a sheet of paper or transparency with a fixed toner image comes in contact for a period of time with a polyvinyl chloride (PVC) surface containing a plasticizer used in making the vinyl material flexible such as for example in vinyl binder covers, and the fixed image adheres to the PVC surface.
There is a need for a toner resin which has low fix temperature and high offset temperature (or wide fusing latitude), and superior vinyl offset property, and processes for the preparation of such a resin. Toners which operate at lower temperatures would reduce the power needed for operation and increase the life of the fuser roll and the high temperature fuser roll bearings. Additionally, such low melt toners (i.e., toners having a MFT lower than 200.degree. C., preferably lower than 160.degree. C.) would reduce the volatilization of release oil such as silicon oil which may occur during high temperature operation and which can cause problems when the volatilized oil condenses in other areas of the machine. In particular, toners with a wide fusing latitude and with good toner particle elasticity are needed. Such toners with wide fusing latitude can provide flexibility in the amount of oil needed as release agent and can minimize copy quality deterioration related to the toner offsetting to the fuser roll.
In order to lower the minimum fix temperature of the binder resin, in some instances the molecular weight of the resin may be lowered. Low molecular weight and amorphous polyester resins and epoxy resins have been used for low temperature fixing toners. For example, attempts to use polyester resins as a binder for toner are disclosed in U.S. Pat. No. 3,590,000 to Palermiti et al. and U.S. Pat. No. 3,681,106 to Burns et al. The minimum fixing temperature of polyester binder resins can be lower than that of other materials, such as styrene-acrylic and styrene-methacrylic resins. However, this may lead to a lowering of the hot offset temperature, and as a result, decreased offset resistance. In addition, the glass transition temperature of the resin may be decreased, which may cause the undesirable phenomenon of blocking of the toner during storage.
To prevent fuser roll offsetting and to increase fuser latitude of toners, various modifications have been made in toner composition. For example waxes, such as low molecular weight polyethylene, polypropylene, etc., have been added to toners to increase the release properties, as disclosed in U.S. Pat. No. 4,513,074 to Nash et al., the entire disclosure of which is hereby totally incorporated by reference herein. However, to prevent offset sufficiently, considerable amounts of such materials may be required in some instances, resulting in detrimental effects such as the tendency to toner agglomeration, worsening of free flow properties and destabilization of charging properties.
Modification of binder resin structure, for example by branching, cross-linking, etc., when using conventional polymerization reactions may also improve offset resistance. In U.S. Pat. No. 3,681,106 to Burns et al., for example, a polyester resin was improved with respect to offset resistance by non-linearly modifying the polymer backbone by mixing a trivalent or more polyol or polyacid with the monomer to generate branching during polycondensation. However, an increase in degree of branching may result in an elevation of the minimum fix temperature. Thus, any initial advantage of low temperature fix may be diminished.
Another method of improving offset resistance is to utilize cross-linked resin in the binder resin. For example, U.S. Pat. No. 3,941,898 to Sadamatsu et al. discloses a toner in which a cross-linked vinyl type polymer is used as the binder resin. Similar disclosures for vinyl type resins are made in U.S. Pat. Nos. Re. 31,072 (a reissue of 3,938,992) to Jadwin et al., 4,556,624 to Gruber et al., 4,604,338 to Gruber et al. and 4,824,750 to Mahalek et al.
While significant improvements can be obtained in offset resistance and entanglement resistance, a major drawback may ensue in that with cross-linked resins prepared by conventional polymerization (that is, cross-linking during polymerization using a cross-linking agent), there exist three types of polymer configurations: a linear and soluble portion called the linear portion, a portion comprising highly cross-linked gel particles which is not soluble in substantially any solvent, e.g., tetrahydrofuran, toluene and the like, and is called gel, and a cross-linked portion which is low in cross-linking density and therefore is soluble in some solvents, e.g., tetrahydrofuran, toluene and the like, and is called sol. The presence of highly cross-linked gel in the binder resin increases the hot offset temperature, but at the same time the low cross-link density portion or sol increases the minimum fix temperature. An increase in the amount of cross-linking in these types of resins results in an increase not only of the gel content, but also of the amount of sol or soluble cross-linked polymer with low degree of cross-linking in the mixture. This results in an elevation of the minimum fix temperature, and as a consequence, in a reduction or reduced increase of the fusing latitude. Also, a drawback of embodiments of cross-linked polymers prepared by conventional polymerization is that as the degree of cross-linking increases, the gel particles or very highly cross-linked insoluble polymer with high molecular weight grow larger. The large gel particles can be more difficult to disperse pigment in, causing the formation of unpigmented toner particles during pulverization, and toner developability may thus be hindered. Also, compatibility with other binder resins may be relatively poor and toners containing vinyl polymers often show vinyl offset.
Cross-linked polyester binder resins prepared by conventional polycondensation reactions have been made for improving offset resistance, such as for example in U.S. Pat. No. 3,681,106 to Burns et al. As with cross-linked vinyl resins, increased cross-linking as obtained in such conventional polycondensation reactions may cause the minimum fix temperature to increase. When cross-linking is carried out during polycondensation using tri- or polyfunctional monomers as cross-linking agents with the polycondensation monomers, the net effect is that apart from making highly cross-linked high molecular weight gel particles which are not soluble in substantially any solvent, the molecular weight distribution of the soluble part widens due to the formation of sol or cross-linked polymer with a very low degree of cross-linking, which is soluble in some solvents. These intermediate high molecular weight species may result in an increase in the melt viscosity of the resin at low and high temperature, which can cause the minimum fix temperature to increase. Furthermore, gel particles formed in the polycondensation reaction which is carried out using conventional polycondensation in a reactor with low shear mixing can grow rapidly with increase in degree of cross-linking. As in the case of cross-linked vinyl polymers using conventional polymerization reactions, these large gel particles may be more difficult to disperse pigment in, resulting in unpigmented toner particles after pulverization, and thus hindering developability.
U.S. Pat. No. 4,533,614 to Fukumoto et al. discloses a loosened cross-linked polyester binder resin which shows low temperature fix and good offset resistance. Metal compounds were used as cross-linking agents. Similar disclosures are presented in U.S. Pat. No. 3,681,106 and Japanese Laid-Open Patent Applications Nos. 94362/1981, 116041/1981 and 166651/1980. As discussed in the '614 patent, incorporation of metal complexes, however, can influence unfavorably the charging properties of the toner. Also, in the case of color toners other than black (e.g., cyan), metal complexes can adversely affect the color of pigments. It is also known that metal containing toner can have disposal problems in some geographical areas, such as for example in the State of California, U.S.A. Metal complexes are often also expensive materials.
Many processes are known for effecting polymerization reactions, including reactive extrusion processes, for both initial polymerization reactions employing monomers or prepolymers, and for polymer modification reactions, such as graft, coupling, cross-linking and degradation reactions. However, it is believed that the prior art does not disclose the use of a reactive extrusion process to prepare cross-linked resins for use in toners.
U.S. Pat. No. 4,894,308 to Mahabadi et al. and U.S. Pat. No. 4,973,439 to Chang et al., for example, disclose extrusion processes for preparing electrophotographic toner compositions in which pigment and charge control additive were dispersed into the binder resin in the extruder. However, in each of these patents, there is no suggestion of a chemical reaction occurring during extrusion.
An injection molding process for producing cross-linked synthetic resin molded articles is disclosed in U.S. Pat. No. 3,876,736 to Takiura in which polyolefin or polyvinyl chloride resin and cross-linked agent were mixed in an extruder, and then introduced into an externally heated reaction chamber outside the extruder wherein the cross-linking reaction occurred at increased temperature and pressure, and at low or zero shear.
In U.S. Pat. No. 4,089,917 to Takiura et al., an injection molding and cross-linking process is disclosed in which polyethylene resin and cross-linking agent were mixed in an extruder and reacted in reaction chambers at elevated temperature and pressure. Heating of the resin mixture occurred partially by high shear in inlet flow orifices. However, the cross-linking reaction in this process still took place in the reaction chambers at low or zero shear, and the final product is a thermoset molded part, and thus is not useful for toner resins.
A process for dispensing premixed reactive precursor polymer mixtures through a die for the purposes of reaction injection molding or coating is described in U.S. Pat. No. 4,990,293 to Macosko et al. in which polyurethane precursor systems were cross-linked in the die and not in the extruder. The dimensions of the die channel were determined such that the value of the wall shear stress was greater than a critical value in order to prevent gel buildup and consequent plugging of the die. The final product is a thermoset molded part, and thus is not useful for toner resins.
It should be noted that the processes disclosed in U.S. Pat. Nos. 3,876,736, 4,089,917 and 4,990,293 are not reactive extrusion processes, because the cross-linking in each case occurs in a die or a mold, and not in an extruder, and the cross-linking takes place at low or zero shear. These processes are for producing engineering plastics such as thermoset materials which cannot be remelted once molded, and thus are not suitable for toner application.